Coral Neuropeptides

Neuropeptides are short linear polypeptides secreted by neurons. Generally, neuropeptides are derived from larger precursor molecules called preprohormones. Preprohormones are post-translationally processed by proteolytic cleavage and modified to yield mature, small amidated peptides (for more details, see https://doi.org/10.3389/fendo.2014.00150; https://www.sysy.com/Neuropeptides)

(Figure from https://doi.org/10.3389/fendo.2020.00339). Schematic representation of eleven preprohormones in Hydra. (A) Preprohormone-A contains unprocessed Hydra-RFamide I, II, III, IV, V, and VI. Preprohormone-B contains unprocessed Hydra-RFamide I, II, V, VII, and VIII. Preprohormone-C contains Hydra-RFamide I and two putative neuropeptide sequences (pQWFSGRFa and pQWLSGRFa). (B) The GLWamide precursor contains unprocessed GLWamides (Hym-53, 54, 248, 249, 331, and 370) and two putative neuropeptide sequences (Hydra-LWamide VI and VIII). (C) The Hym-176 precursor (Hym-176A) contains one copy of unprocessed Hym-176 and Hym-357. Hym-176B contains Hym-357 and one putative neuropeptide (KPLKVMKM). Hym-176C and Hym-176D contain one copy of Hm-176-homologous peptide (Hym-176C and Hym-176D), respectively. Hym-176C also contains one unprocessed Hym-690. Hym-176E contains one putative neuropeptide sequence (NPFIFKGHKH). (D) The Hym-355 precursor contains one unprocessed Hym-355. (E) The FRamide precursor contains unprocessed FRamide-1 and−2. Black box: signal sequence, a: amide.

One of the important neuropeptides identified in cnidarians is the glycine-leucine-tryptophan-amide family neuropeptides (GLWamides, also referred to as LWamides). GLWamides were first isolated from the sea anemone Anthopleura elegantissima as a trigger for metamorphosis in Hydractinia echinata larvae. Seven types of GLWamides were later isolated from Hydra magnipapillata. Some of these GLWamides have been shown to be involved in a wide range of physiological functions in cnidarians, including larval metamorphosis, muscle contraction (myoactivity), polyp detachment, and oocyte maturation and ovulation.

In the coral, Fimbriaphyllia (Euphyllia) ancora, GLWamide neurons were mainly distributed in the epidermis of the mouth region and tentacle.

(For more details, see Shikina et al., 2020, https://www.nature.com/articles/s41598-020-66438-3)

Treatment of polyps with synthetic GLWamide peptide (PPGLWamide) induced the contraction of coral polyps. These results suggest that GLWamides are involved in polyp contraction (myoactivity) in adult scleractinians. 

(For more details, see Shikina et al., 2020, https://www.nature.com/articles/s41598-020-66438-3)

Deduced amino acid sequence of the EaRFamide preprohormone (a) and the twenty potential peptides predicted from the sequence using Peptraq software (b). In (a) framed amino acids indicate the signal peptide. KR, predicted dibasic cleavage sites; GR, C-terminal amidation with a monobasic cleavage site; underline, location of potential RFamide peptide sequences; asterisk, QGRF motif. In (b) the potential peptides are listed to the left of each column, and the positions of each peptide are shown to the right (aa residue).

(For more details, see Zhang et al., 2021, https://doi.org/10.1016/j.ygcen.2021.113905)

Effects of QGRFamide peptide treatment on isolated tentacles. Isolated tentacles (4 tentacles are connected) and single isolated tentacles (an isolated tentacle) were treated with 10 μM synthetic QGRFamide peptide or vehicle (H2O). The treated tentacles were photographed before treatment and 10 min after treatment. Note that the tentacles treated with 10 μM QGRFamide peptide exhibited distinct contraction. 

(For more details, see Zhang et al., 2021, https://doi.org/10.1016/j.ygcen.2021.113905)